ABSTRACT Nearly 40% of homeless adults currently struggle with an alcohol use disorder (AUD). While ?housing first? programs are increasingly available, most homeless people must refrain from alcohol use to obtain and maintain housing. Contingency management (CM) is one of the most effective behavioral interventions for initiating abstinence from alcohol and drugs. In a randomized trial of CM for AUDs in adults with co-occurring serious mental illness, 60% of whom were homeless, we found that those who received CM were 3 times more likely to submit alcohol-negative urine ethyl glucuronide tests (uEtG), relative to controls. However, homeless individuals were 8 times more likely to drop out of CM, relative to housed individuals. Therefore, CM appears to work best for those who are housed. By using CM to reduce alcohol use in formerly homeless individuals who are now housed, CM might prevent subsequent alcohol-associated homelessness. The brief periods of detection of uEtG (2-5 days) and other alcohol biomarkers require monitoring of abstinence multiple times a day (breath tests) or week (uEtG) in a CM intervention. Such frequent monitoring is not feasible in most housing programs or necessary once individuals obtain prolonged periods of abstinence. Phosphatidylethanol (PEth) is a lipid-based biomarker that can detect alcohol use for up to 28 days. The lengthy detection period of PEth would allow for a more feasible monitoring and reinforcement schedule (e.g. 1 assessment every 1 to 4 weeks) and allows for the development of a CM intervention that can reinforce maintenance of long-term abstinence (e.g., monitoring and reinforcement once every 4 weeks). We propose to assess the feasibility and initial efficacy of a PEth-based CM intervention by randomizing 50 currently housed, formerly homeless adults with AUDs receiving supported housing from Catholic Charities Spokane. Participants will be randomized to either 6 months of a) treatment-as- usual (TAU) and reinforcers for submitting blood samples, regardless of PEth results (Non-contingent Control), or b) TAU and reinforcers for PEth results consistent with abstinence (CM). They will then complete a 3-month post-intervention follow-up. Our CM intervention will include 2 phases. In the initiation phase, PEth samples will be collected and reinforcers delivered weekly until individuals attain a PEth level consistent with prolonged abstinence (i.e., PEth 16:0/18:1 <20 ng/mL). CM participants will then enter the maintenance phase where they will submit less frequent PEth samples, 1 sample every 2 to 4 weeks. In this treatment development study, we will simultaneously evaluate intervention acceptability and feasibility using quantitative and qualitative methods and initial efficacy by documenting differences in alcohol abstinence, housing tenure and alcohol associated harms. We will use the Theoretical Domains Framework to guide our assessment of factors that might influence implementation of the intervention. Results will be used to support an R01 application to conduct a multi-site hybrid effectiveness/implementation trial to determine the impact of the intervention on alcohol use, housing outcomes, and alcohol related harms, as well as assess implementation science outcomes.